1. Field of The Invention
The present invention relates generally to a process for forming a copper wiring. Specifically, the present invention relates to a process for protecting a copper wiring formed on a semiconductor device from oxidation while forming.
2. Description of The Background Art
Recently, a wiring formed on a semiconductor device becomes minimized according to high integration of Ultra Large Scale Integrated Circuits (ULSI). Materials selected from aluminum and aluminum alloy has been widely utilized for such wiring. However, durability of such wiring against electromigration decreases according to minimization of interwiring distance. This causes deterioration of the device reliability. Additionally, aspect ratio of the wiring is increased when utilizing aluminum. This causes processing difficulty thereafter, such as forming an insulating film and flattening the wiring. In order to reduce aspect ratio of the wiring, metals having high melting point, such as molybdenum (Mo) and tungsten (W), are utilized. However, resistance of such metals becomes over 2 times of that of aluminum, especially, becomes higher when the wiring is formed of a very thinned layer. Therefore, materials which have sufficient durability against electromigration and low resistance have been required.
Copper (Cu) has about 1.4 .mu. .OMEGA. cm of resistivity lower than that of aluminum which is about 2.8 .mu. .OMEGA. cm. Therefore, a copper wiring can be formed of a thinner film allowing aspect ratio thereof to be reduced. Moreover, the copper wiring has sufficient durability against electromigration because of its low resistivity. Thus, such wiring may be provided with a micro device.
However, there are some problems when using copper for a wiring. First, copper will easily suffer from oxidation in an atmosphere including minimum percentage of oxygen at a temperature of about 200.degree. C. Additionally, wiring surfaces are altered to copper oxide during the steps of resist ashing or washing. Secondly, an oxide film formed on the copper surface does not become sufficiently chemically stabilized. Therefore, oxygen molecules are dispersed in the copper film when contacting with the wiring surface, then copper in the interior of the film is altered to copper oxide which permits a rapid increase of electrical resistant thereof. Thus, copper becomes impracticable as a material for a wiring.
In order to solve the aforementioned problems, Japanese Patent First Publication (not allowed) No. 63-299250 discloses a process for forming an anti-oxidized copper wiring by depositing silicon on a surface of a copper wiring. Silicon is deposited by sputtering on a film of silicon dioxide (SiO.sub.2) on which the copper wiring is laid. Then, heat treatment of the film is accomplished at temperatures of 800.degree. to 1000.degree. C. to form Cu-SiO.sub.2 alloy on the surface thereof. Thus, a layer of alloy protects the copper from oxidation.
Alternatively, Japanese Patent First Publication (not allowed) No. 63-156341 discloses a process for forming a barrier layer on the surroundings of a copper wiring to protect copper from contacting with oxygen. An oxidation barrier formed of a film of titanium nitride (TiN) is applied to the surroundings of the copper wiring using bias sputtering.
However, both are processes for protecting a copper wiring from oxidation after finishing the wiring patternizing. Therefore, surface oxidation of a copper film meanwhile, such as ashing and washing, cannot be prevented.
Japanese Patent First Publication (not allowed) No. 2-125447 discloses a process for forming an anti-oxidation film having electrical conductivity on a surface of a copper film to protect the surface thereof from oxidation. The copper film is subjected to vapor or liquid including aromatic amines, such as benzotriazole, the anti-oxidation film having several molecules of thickness is laid on the copper film. Then, a wiring pattern is formed on the copper film via the anti-oxidation film. Patternization of a copper wiring is done by etching the wiring pattern. Then, after removing the surroundings, exposed surfaces of the copper wiring to ambient atmosphere are subjected to the vapor for the second time, similarly to the previously mentioned, vapor to coat residual surroundings of the copper wiring with the anti-oxidation film.
However, applying anti-oxidation film on the copper film must be done twice, i.e., before and after patternizing the copper wiring to provide sufficient protection thereof. This complicates manufacturing process. Furthermore, coating on the surroundings of the wiring decreases reliability thereof.